1. Field of the Invention
The present invention relates to an improved method for producing hydrogen peroxide continuously by repeatedly reducing and oxidizing anthraquinone compounds (hereinafter this refers to an anthraquinone and/or a tetrahydroanthraquinone) contained in a working solution. More specifically, the present invention relates to a method for producing hydrogen peroxide characterized in that specified amounts of ethylanthraquinone and ethyltetrahydroanthraquinone are used as the anthraquinone compounds.
2. Description of the Related Art
An anthraquinone process is known as an industrial hydrogen peroxide production process. In this process, an anthraquinone compound is dissolved in an organic solvent to obtain a working solution, and in a hydrogenation step the anthraquinone compound is reduced with hydrogen in the presence of a hydrogenation catalyst so as to produce an anthrahydroquinone compound. Next, in an oxidation step the anthrahydroquinone compound is converted back into the anthraquinone compound, with hydrogen peroxide being produced at the same time. The hydrogen peroxide in the working solution is separated out from the working solution by a process such as water extraction. The working solution from which the hydrogen peroxide has been extracted is returned into the hydrogenation step, whereby a cyclic process is formed.
The hydrogenation step is the most important step in the anthraquinone process, and as a characteristic feature can be broadly classified according to whether an anthraquinone (hereinafter this refers to either a substituted anthraquinone or unsubstituted anthraquinone) is used, or a tetrahydroanthraquinone (hereinafter this refers to either a substituted tetrahydroanthraquinone or unsubstituted tetrahydroanthraquinone) is used, or both are used as a reaction medium.
Prior art in which an anthraquinone or a tetrahydroanthraquinone is used as the reaction medium is collected together in Japanese Patent Application Laid-open No. 6-191803. That is, hitherto, problems have been cited that in the case that a tetrahydroanthraquinone is used as the reaction medium, the reaction conditions in the oxidation step must be strengthened, whereas in the case that an anthraquinone is used as the reaction medium, the solubility of the anthrahydroquinone produced through the hydrogenation is low. As a method for producing hydrogen peroxide efficiently and economically, there has thus been proposed a hydrogen peroxide production process in which in the reduction step all of a tetrahydroanthraquinone and some or all of an anthraquinone in a working solution are reduced, and the content of an anthrahydroquinone in the working solution after the reduction step is kept higher than the content of a tetrahydroanthrahydroquinone. That is, in Japanese Patent Application Laid-open No. 6-191803, there is proposed a process according to which the amount of hydrogen peroxide obtained per cycle is kept high. Moreover, by using more of the anthrahydroquinone for which the oxidation reaction proceeds readily than the tetrahydroanthrahydroquinone for which the oxidation reaction rate is slow, the energy consumption in the oxidation step can be kept down.
Moreover, in Japanese Patent Application Laid-open No. 6-191803, a case in which two or more anthraquinone compounds are used is introduced, the use of such a mixture of anthraquinone compounds having been known from long ago.
In Japanese Patent Application Laid-open No. 2001-163608 and Published Japanese Translation of PCT Application No. 2002-511377, a case that mainly a tetrahydroanthraquinone is used as the reaction medium is introduced. In Japanese Patent Application Laid-open No. 2001-163608, a case is shown in which a mixture of an ethylanthraquinone and an amylanthraquinone is used as the anthraquinone compounds, and in Published Japanese Translation of PCT Application No. 2002-511377, a case is introduced in which a mixture of an ethylanthraquinone and novel compounds 2-(4-methyl-3-pentenyl)-anthraquinone and 2-(4-methylpentyl)-anthraquinone is used. These Japanese Patent Application Laid-open No. 2001-163608 and Published Japanese Translation of PCT Application No. 2002-511377 are very similar in that two or more different anthraquinone compounds are used, and mainly a tetrahydroanthraquinone is used as the reaction medium; the difference is the substituents of the anthraquinone compounds used. Moreover, a characteristic feature is that the amount of hydrogen peroxide obtained per cycle is kept high due to using mainly a tetrahydroanthraquinone as the reaction medium.
However, in the case of using mainly a tetrahydroanthraquinone as the reaction medium, strengthening the conditions in the oxidation step for the tetrahydroanthrahydroquinone which has a slow oxidation reaction rate is a problem as described in Japanese Patent Application Laid-open No. 6-191803. Moreover, the hydrogen peroxide obtained through such a hydrogen peroxide production process using mainly a tetrahydroanthraquinone as the reaction medium has therein a very large amount of decomposition product originating from the tetrahydroanthraquinone. This is decomposed matter having as a main component thereof an alkylated phthalic acid compound originating from the tetrahydroanthraquinone or a tetrahydroanthraquinone epoxide which is an oxidation product of the tetrahydroanthraquinone. The grounds for being originating from the tetrahydroanthraquinone are that the phthalic acid compound obtained is all an alkylated phthalic acid compound originating from the reaction medium. That is, in the case of using 2-ethyltetrahydroanthraquinone as the reaction medium, this generally mainly comprises a β-tetrahydroanthraquinone, and ethylphthalic acid is present in the hydrogen peroxide obtained. Moreover, such alkylated phthalic acid compounds are hardly seen in hydrogen peroxide obtained using a hydrogen peroxide production process in which an anthraquinone compound as described in Japanese Patent Application Laid-open No. 6-191803 is used as the reaction medium. That is, such decomposed matter is a problem due to being present as impurities in the product hydrogen peroxide, this depending on the amount of the tetrahydroanthraquinone in the working solution.
Such impurities can be separated out industrially by any of various processes, but even in the case, for example, of distillation or removal by adsorption, if the amount of impurities in the hydrogen peroxide used as the starting material is high, then the apparatus must be correspondingly large and hence the equipment cost burden becomes high, and moreover the equipment operating time becomes long, and hence this is clearly undesirable. Moreover, when removing the impurities, loss or decomposition of the hydrogen peroxide itself may occur, and hence the burden on the production amount and variable costs cannot be ignored.
Moreover, there are various fields in which the hydrogen peroxide produced may be used, but in any of these fields, for example bleaching of paper pulp, addition to food, or an electronics industry cleaning solution, it is clear that low-impurity hydrogen peroxide is preferable. In particular, hydrogen peroxide for the electronics industry is generally known as ultra-pure hydrogen peroxide, being hydrogen peroxide that has been subjected industrially to special purification, and the amount of impurities in the starting material hydrogen peroxide is a burden as is on the purification process, which is undesirable, and moreover leads to an increase in the amount of impurities in the ultra-pure hydrogen peroxide obtained through the purification, which is undesirable.
Meanwhile, with the process described in Japanese Patent Application Laid-open No. 6-191803, more anthraquinone than tetrahydroanthraquinone is used for the reaction medium, and hence as described above, it is clear that the amount of impurities which comprise mainly an alkylated phthalic acid compound is lower than in the case of using mainly tetrahydroanthraquinone as the reaction medium. However, it is clearly stated in Japanese Patent Application Laid-open No. 6-191803 that it is preferable to use amylanthraquinone as the anthraquinone, and in addition to this an alkylated anthraquinone, i.e. ethylanthraquinone or tertiary-butyl-anthraquinone, or a mixture of a plurality of anthraquinones can be used, but it is not made clear that it is advantageous to use any specific mixture. For example, in the case of using amylanthraquinone whose use is clearly stated as being preferable, with the process of Japanese Patent Application Laid-open No. 6-191803, it is a fact that the reaction rate in the hydrogenation step is reduced. That is, in the hydrogenation step, to make up for the slow reaction, a large reactor is required, and hence the equipment cost burden is increased. Moreover, as another solution, the amount of a catalyst may be increased so as to make up for the slow reaction rate, but not only is purchasing the catalyst an initial investment, moreover there is a problem that upon increasing the amount of the catalyst, the amount of leaching out of metal from the catalyst increases. The metal leaching out from the catalyst may be, for example, palladium, platinum or nickel, and these not only bring about decomposition of the hydrogen peroxide produced in the oxidation step, but moreover leach out as impurities in the hydrogen peroxide produced, so as to become a problem as described above.
Overall, there is a problem that with the process described in Japanese Patent Application Laid-open No. 6-191803, inorganic impurities such as metal contaminate the hydrogen peroxide produced, and with the process described in Japanese Patent Application Laid-open No. 2001-163608 or Published Japanese Translation of PCT Application No. 2002-511377, organic impurities contaminate the hydrogen peroxide produced.